AS SOON AS THE IDENTITY of Luigi Mangione, the man alleged to have shot and killed UnitedHealthcare CEO Brian Thompson, was revealed, the online hunt—a familiar collective ritual—began. The basic dossier came together fast: Mangione had accounts on X, on Instagram, Facebook, LinkedIn, and Goodreads. There was, maybe, a Tinder profile; finding his Reddit took a day. Reporters and social-media users noted possible retrospective red flags, eerie fragments of information, and small ironies. On Goodreads, Mangione had posted a contrarian, and favorable, response to Ted Kaczynski’s manifesto. Was it evidence of violent tendencies? He’d reviewed a couple of books about back pain, and on Reddit, he wrote about suffering from a spinal condition. He was a member of a sub-Reddit devoted to efficient backpacks: Had he been planning in plain…